Did you buy a dial-back timing light?

Perhaps a Penske from Sears?

And does your car run an MSD ignition amplifier?
(Other brand amplifiers may be the same.)

I bought a dial-back light and found the timing showed way off compared to the non dial-back Penske light I'd been using.

I took the light back, but since then I've found some of the dial-back lights when used with an amplified ignition setup will show off time.